In the parent application for this supplement, R01CA123055, we described specific aims to determine (i) in which cell type(s) COX-2 must be expressed for skin cancer induction, using the DMB/TPA and UV-B protocols, and (ii) in which cell type(s) skin cancer induction can be modulated by COX-2 over expression. We generated "floxed" Cox2 (Cox2flox) mice in which critical Cox2 exons are flanked by loxP sites and COX-2 conditional over expresser (COX-2 COE) transgenic mice in which the CAG promoter drives COX-2 protein expression. However, the CAG promoter is separated from the COX-2 coding region by a floxed transcription/translation STOP sequence in the COX-2 COE mouse. We planned to determine, by crossing Cox2flox mice with mice that express Cre recombinase in a tissue specific fashion, in which cell type(s) COX-2 must be expressed for skin cancer induction by DMBA/TPA and/or UV-B. When the Cox2 gene is deleted in skin epithelial cells (by crossing Cox2flox mice with Keratin14CreTg mice) DMBA/TPA skin tumor formation is extensively inhibited. In contrast - and surprisingly, we think - when the Cox2 gene is deleted in macrophage/monocytes (by crossing Cox2flox mice with LysMCreTg mice) skin tumor formation is increased, relative to littermate controls. COX-2 mediated prostaglandin (PG) formation in epithelial cells and in monocyte/macrophages has distinctly different roles in skin cancer development. In light of our data demonstrating distinct roles for COX-2 mediated PG expression in different cells during skin cancer progression, we add a third goal in this supplement application: We wish to ask "Can COX-2 expression in a single cell type, from the endogenous Cox2 promoter, mediate/modulate DMBA/TPA or UV-B induced skin cancer?" For example, can "endogenous" COX-2 expression from epithelial cells, without COX-2 expression in other cells, suffice for DMBA/TPA skin cancer induction? To answer this question we will develop Cox2 cell-restricted (Cox2Cr) mice in which the endogenous Cox2 gene is functionally silenced, unless a suppressing STOP sequence is removed by cell type-specific Cre excision. PUBLIC HEALTH RELEVANCE: COX-2 inhibitors are still investigated as therapeutic and preventive agents for epithelial cancers, despite cardiovascular side effects. "Down-stream" COX-2 pathway effectors (prostanoid synthases and receptors) have become targets for similar research. If we can pinpoint the cell type(s) in which COX-2 expression and the prostanoid effectors regulate cancer development, we may be able to target those steps with lower concentrations and less prolonged application of pharmacologic agents. We anticipate we will be able to define the critical cells for COX-2 cancer enhancement and suppression.